Theories about coil-shielding

[Henrikras]

Funny...I allso just ordred some Graphite powder at Ebay and want to try it with some varnish or Maybee another solution.

[mykaitch]

Without a screen, it may be thet as you offer the coil head to the ground the E field around the The Tx coil is coupled into the Rx coil - using the permitivity of the soil as the capacitive coupling medium. It is this efield coupling of Tx to Rx using the ground as the dielectric coupler from Tx to Rx coil - so the Rx coil gets a voltage introduced into it which make you false? Maybe.

I think you are right. I did a lot of work for 3M to do with ground capacitance (for car parking/object sensing) and it os a very real and measurable effect.
The RFI spray arrived today. I sprayed my case and dropped in the concentric coils. There was a small change in the near perfect NULL. After trimming that out I am left with <1mV. So, proof then; foil is no good for screening a concentric coil. All the articles I have read use graphite but none say why - now we know. I also bought a very cheap bag of graphite dust on Ebay. I am going to mix it with varnish and see what it does.

[golfnut]

Without a screen, it may be thet as you offer the coil head to the ground the E field around the The Tx coil is coupled into the Rx coil - using the permitivity of the soil as the capacitive coupling medium. It is this efield coupling of Tx to Rx using the ground as the dielectric coupler from Tx to Rx coil - so the Rx coil gets a voltage introduced into it which make you false? Maybe.

[mykaitch]

There is said some wise words here... http://www.findmall.com/read.php?34,1174380
and more here

http://www.geotech1.com/forums/showthread.php?15791-Crude-coil-sheilding-paint

http://www.geotech1.com/forums/showthread.php?20189-coil-shielding

and you can find more... I think all the quenstretion we got, can Be found in some earlyer threads.
Im still learning.. But would say yes to you, you must make the shielding with graphite spray or powder.
But you need to find out how you make and where you connect the drain wire.
Henrik

I have some copper tape that has a conductive glue backing. Pressed down onto graphite, alu etc it makes an <1R connection. Didn't realise you had to shield a DD too but mine works great unless I hit a root or something. As for null, well you really can get <1mV with patience. As the epoxy dried I monitored it on the scope and made the very finest of adj until it set. I too found that a small peice (in my case 2cm sq copper fol) was the final tweak for my concentric. I experimented with diff shapes/sizes but for me, 2cm sq was best.

[Henrikras]

There is said some wise words here... http://www.findmall.com/read.php?34,1174380
and more here

http://www.geotech1.com/forums/showthread.php?15791-Crude-coil-sheilding-paint

http://www.geotech1.com/forums/showthread.php?20189-coil-shielding

and you can find more... I think all the quenstretion we got, can Be found in some earlyer threads.
Im still learning.. But would say yes to you, you must make the shielding with graphite spray or powder.
But you need to find out how you make and where you connect the drain wire.
Henrik

[maikl]

Most factory manufacturer of metal detectors used graphite as a shield for the coil...

[mykaitch]

You Can make the nicest coil in the World, with the best Null and Q factor and everything. Its perfect in the lab... But it worthless with out or with the wrong shielding outside.

I think I get you. You mean I MUST shield it ? I know it is great in air tests, still waiting for graphte stuff. Do you agree with my guess that the copper/foil shield is no good for lining the case of a concentric ? I have found that some manufacturers do not shield the case at all.

[Henrikras]

You Can make the nicest coil in the World, with the best Null and Q factor and everything. Its perfect in the lab... But it worthless with out or with the wrong shielding outside.

[mykaitch]

I thought the idea is it electrically shields but not magnetically. You need mu metal to stop a magnetic field.
I agree though it does reduce the field somewhat so best to avoid it!

So it is, but finally, after a year trying, I have a superb concentric head.What has this to do with the thread?
Well, I made a DD - very easy, and I wrapped the Ds in foil and copper tape as an RFI shield - yes,we need the magnetism.
For a year now, I have made at least 20 concentric coils that were crap - always I lined the shell with Alu and copper.
This one now is different. (I will be posting full details). The null is about 0.3 mV (point 3!!!)
When put into the foil lined case - CRAP!
I am waiting for an RF graphite aerosol to arrive - notice that ALL DIY concentrics use this method and NOT foil.
DDs are quite different to Cons which is why foil works for them.

[Silver Dollar]

I thought the idea is it electrically shields but not magnetically. You need mu metal to stop a magnetic field.
I agree though it does reduce the field somewhat so best to avoid it!

[Funfinder]

Some say coils must be shielded, others not.

What is a shielded coil?

It is a 2 overlaping antenna-solution, where the shielding works as a half-permeable specialized antenna.

Seen from the construction the coil is a magnetical-loop antenna
while the shielding consists just in one or two full or single loops.

One thing must be clear:
every shielding also shields the power of the magnetical-loop, may it just for 10%
and because this shielding is such close its like a very weak-shortcut for the coils EM-field.


The main question:
What must be shielded at all? The grounds capacity?!
What capacity, the ground is not a battery, its grounded, its zero!

The electrostatic voltage, coming from the coils plastic housing or the natural sky-ground potential?


Its logical clear that every shielding will reduce the power and sensitivity of the coil so we should find
a way to get rid of possible disturbing currents or EM-fields with better methods and more direct ways.

Shielding the electronic housing (by little metal boxes etc) is OK and common for all kind or receivers.